Thermal transfer printing has displaced impact printing in many applications due to advances such as the relatively low noise levels which are attained during the printing operation. Thermal transfer printing is widely used in special applications such as in the printing of machine readable bar codes and magnetic alpha-numeric characters. The thermal transfer process provides great flexibility in generating images and allows for broad variations in style, size and color of the printed image. Representative documentation in the area of thermal printing includes the following patents:
U.S. Pat. No. 3,663,278, issued to J. H. Blose, et al. on May 16, 1972, discloses a thermal transfer medium comprising a base with a coating comprising of cellulosic polymer, thermoplastic aminotriazine-sulfonamide-aldehyde resin, plasticizer and a "sensible" material such as a dye or pigment.
U.S. Pat. No. 4,315,643, issued to Y. Tokunaga et al. on Feb. 16, 1982, discloses a thermal transfer element comprising a foundation, a color developing layer and a hot melt ink layer. The ink layer includes heat conductive material and a solid wax as a binder material.
U.S. Pat. No. 4,403,224, issued to R. C. Winowski on Sep. 6, 1983, discloses a surface recording layer comprising a resin binder, a pigment dispersed in the binder, and a smudge inhibitor incorporated into and dispersed throughout the surface recording layer, or applied to the surface recording layer as a separate coating.
U.S. Pat. No. 4,463,034, issued to Y. Tokunaga et al. on Jul. 31, 1984, discloses a heat-sensitive magnetic transfer element having a hot melt or a solvent coating.
U.S. Pat. No. 4,628,000, issued to S. G. Talvalkar et al. on Dec. 9, 1986, discloses a coating formulation that includes an adhesive-plasticizer or sucrose benzoate transfer agent and a coloring material or pigment.
U.S. Pat. No. 4,687,701, issued to K. Knirsch et al. on Aug. 18, 1987, discloses a heat sensitive inked element using a blend of thermoplastic resins and waxes.
U.S. Pat. No. 4,707,395, issued to S. Ueyama et al., on Nov. 17, 1987, discloses a substrate, a heat-sensitive releasing layer, a coloring agent layer, and a heat-sensitive cohesive layer.
U.S. Pat. No. 4,777,079, issued to M. Nagamoto et al. on Oct. 11, 1988, discloses an image transfer type thermosensitive recording medium using thermosoftening resins and a coloring agent.
U.S. Pat. No. 4,778,729, issued to A. Mizobuchi on Oct. 18, 1988, discloses a heat transfer sheet comprising a hot melt ink layer on one surface of a film and a filling layer laminated on the ink layer.
U.S. Pat. No. 4,923,749, issued to Talvalkar on May 8, 1990, discloses a thermal transfer ribbon which comprises two layers, a thermosensitive layer and a protective layer, both of which are water based.
U.S. Pat. No. 4,975,332, issued to Shini et al. on Dec. 4, 1990, discloses a recording medium for transfer printing comprising a base film, an adhesiveness improving layer, an electrically resistant layer and a heat sensitive transfer ink layer.
U.S. Pat. No. 4,983,446, issued to Taniguchi et al. on Jan. 8, 1991, describes a thermal image transfer recording medium which comprises as a main component, a saturated linear polyester resin.
U.S. Pat. No. 4,988,563, issued to Wehr on Jan. 29, 1991, discloses a thermal transfer ribbon having a thermal sensitive coating and a protective coating. The protective coating is a wax-copolymer mixture which reduces ribbon offset.
U.S. Pat. Nos. 5,128,308 and 5,248,652, issued to Talvalkar, each disclose a thermal transfer ribbon having a reactive dye which generates color when exposed to heat from a thermal transfer printer.
U.S. Pat. No. 5,240,781, issued to Obatta et al., discloses an ink ribbon for thermal transfer printers having a thermal transfer layer comprising a wax-like substance as a main component and a thermoplastic adhesive layer having a film forming property.
Thermal transfer ribbons are a common form of thermal transfer media. Most thermal transfer ribbons employ polyethylene terephthalate (PET) polyester as a substrate. The functional layer which transfers ink, also referred to as the thermal transfer layer, is deposited on one side of the substrate and a protective backcoat is deposited on the other side of the polyethylene terephthalate substrate. Untreated polyethylene terephthalate will not pass under a thermal print head without problems. The side of the polyethylene terephthalate substrate which comes in contact with the print head, i.e., the side opposite the thermal transfer layer, must be protected during the printing process. Failure to do so will result in the polyethylene terephthalate sticking to the heating elements during the heating cycle. Polyethylene terephthalate is also an abrasive material which will cause unacceptable wear on the print head. Therefore, conventional thermal transfer ribbons which employ a polyethylene terephthalate substrate treat the backside of the substrate as part of the coating process to form a barrier between the polyethylene terephthalate and the print head. This material is referred to herein as a "backcoat".
The backcoats are usually comprised of silicone polymers. The most common backcoats are silicone oils and UV cured silicones. The silicone oils can be delivered directly to the PET substrate or via an organic solvent. For direct delivery to the web, a multi-roll coater head is used. Multi-roll coating heads are expensive, difficult to operate and often require high coat weights to obtain uniform coverage when compared to solvent-based coating systems. The precursors to UV cured silicones are applied directly to the web, as well, and suffer from the same disadvantages associated with delivering silicone oils directly to the PET substrate coupled with other additional requirements of curing the silicone coating. Forming backcoats with an organic solvent based system allows for the use of simpler coating methods and equipment while providing more uniform coatings at low coat weights. These cost advantages are limited or lost due to the need to reclaim or incinerate the organic solvent removed from the PET substrate. The organic solvents are considered to be environmentally unfriendly and may also create exposure hazards for operators. The energy costs to remove the organic solvent and costs of investment and operation of organic solvent reclaimers and incinerators are significant. Replacing the organic solvents for these silicone oils with water requires the use of an emulsifier. Conventional emulsifiers contribute to increased buildup on the thermal print heads, resulting in increased wear.
A suitable replacement for the silicone oils has not been found. Other materials which can be coated with an aqueous solvent either suffer from the same disadvantages such as requiring an emulsifier which degrades the print head, or they do not provide the performance of the silicone oils, often building up on the print head requiring periodic cleaning.